1. Field of the Invention
The present invention relates to a punch unit for punching a hole (hereinafter referred to as xe2x80x9cpunch holexe2x80x9d) in a soft metal sheet such as an aluminum sheet and the like (hereinafter also referred to as xe2x80x9cworkpiecexe2x80x9d), wherein the punch hole assumes a circular shape or any other suitable shape, and the soft metal sheet having been punched is used in preparing a printing plate in the field of printing plate making.
2. Description of the Related Art
Heretofore, in order to punch a soft metal sheet such as an aluminum sheet and the like used in preparing a printing plate in the field of lithographic printing plate making, a punch unit of a reciprocating type has been used as is in the case where a piece of paper is punched as a workpiece.
However, in case that the soft metal sheet such as the aluminum sheet and the like is punched by using a conventional punch unit heretofore having been used in punching a piece of paper, the conventional punch unit has its punch and its die remarkably shortened in service life. Further, in the case of the conventional punch unit, the soft metal sheet tends to stick to the punch when punched, which impairs the quality of a punch hole of the soft metal sheet thus punched. This is a problem inherent in the conventional punch unit.
In order to solve this problem, it is necessary and important to set a radial clearance between the punch and the die at an appropriate value. However, in the conventional punch unit, a punch guide and a die block thereof are formed into two different components, and then assembled into a die assembly by using suitable fastening means such as rivets, screws and the like. Due to this, it is difficult for the conventional punch unit to precisely control the radial clearance between the punch and the die. When the punch is deviated from its correct position with respect to the corresponding die, a punch hole of the soft metal sheet thus punched becomes poor in quality.
Consequently, it is an object of the present invention to provide a punch unit for punching a precise high-quality punch hole in a soft metal sheet in a steady manner.
In accordance with a first aspect of the present invention, the above object of the present invention is accomplished by providing:
In a punch unit of a reciprocating type provided with a punch and a die both adapted to punch a punch hole in a soft metal sheet, wherein the punch is slidably guided by a punch guide and reciprocally driven in a condition in which the die cooperates with the punch, the improvement wherein: the punch guide and the die are integrally formed into a block.
As described above, in the conventional punch unit, since the die unit is constructed of two different components, that is, the punch guide and the die block which are assembled into the die assembly by using suitable fastening means such as rivets, screws and the like, it is difficult to precisely control the radial clearance between the punch and the die when the soft metal sheet is punched. Further, in the conventional punch unit, there is a fear that the punch deviates from its correct position with respect to the die since the die assembly assembled from two different components is relatively poor in rigidity.
In contrast with this, in the punch unit of the present invention, such two different components, that is, the punch guide and the die are integrally formed into a single block. Due to this, the punch unit of the present invention is excellent in rigidity, which solves the above fear inherent in the conventional punch unit.
In accordance with a second aspect of the present invention, the above object of the present invention is accomplished by providing:
the punch unit as set forth in the first aspect of the present invention, wherein the punch guide comprises:
a lower guide for guiding the punch in the side of the die, wherein the lower guide is provided with a through-hole window, the through-hole window extending in a direction perpendicular to a longitudinal axis of a bore of the lower guide, whereby an upper portion of the through-hole window forms the lower guide and a lower portion of the through-hole window forms a gate, the gate serving as a part of the cleaning means for removing the fragment of the soft metal sheet from the punch in the punching operation; and
an upper guide for guiding the punch in its driven side.
In the punch unit of the present invention has the above construction, when the punch is reciprocally driven in a vertical plane to punch a punch hole in the soft metal sheet, the fragment of the soft metal sheet having stuck to the punch is scraped off the punch by the cutting edge portion of the gate and discharged outward through a workpiece passage and the through-hole window. Consequently, in the punching operation, it is possible to reduce a frictional drag of the punch caused by such fragment of the soft metal sheet. This also improves the punch hole in quality. Since the gate serves as a cleaning blade during the punching operation, it is possible to reduce the gate in thickness.
Preferably: the soft metal sheet has a thickness of less than or equal to 0.5 mm; and, a radial clearance between the punch and the die is set at a value of from 4 to 15% of a thickness of the soft metal sheet.
In the conventional punch unit for punching a piece of paper and the like, a radial clearance between the punch and the die is set at a value of from 5 to 10 xcexcm. Consequently, when a soft metal sheet such as an aluminum sheet and the like, has a thickness less than 0.5 mm is punched by using the conventional punch unit, a secondary shearing action occurs in the punch hole to have the soft metal sheet often stick to the punch, which prevents the punch from producing a high-quality punch hole and considerably reduces the service life of each of the punch and the die.
In view of the above problems, in the punch unit of the present invention, when the sheet has a thickness less than 0.5 mm is punched, a radial clearance between the punch and the die is set at a value of from 4 to 15% of a thickness of the soft metal sheet, which enables the punch unit of the present invention to punch a high-quality punch hole in the soft metal sheet under its shearing action. The reason why the radial clearance between the punch and the die is set at the above value of from 4 to 15% of a thickness of the soft metal sheet is as follow: namely, when the radial clearance is less than or equal to 4% of the thickness of the soft metal sheet, the punch tends to gall the die. On the other hand, when the radial clearance is more than 15% of the thickness of the soft metal sheet, burrs are produced in the punch hole. Preferably, the radial clearance between the punch and the die should be set at a value of from 6 to 12% of the thickness of the soft metal sheet. In case that the soft metal sheet has a thickness of 0.3 mm for example, the radial clearance should be set at a value of 30xc2x15 xcexcm.
Preferably, the punch unit of the present invention is further provided with a sheet hold-down means for holding down the soft metal sheet when the soft metal sheet is punched, which improves the punch hole in quality.
After the soft metal sheet is punched, the soft metal sheet thus punched tends to stick to the punch due to its softness when the punch is retracted from the soft metal sheet. Such sticking action of the soft metal sheet to the punch often results in formation of burrs, wherein such burrs assume a lampshade-like shape as a whole and project outward from a peripheral portion of the punch hole. This prevents the punch hole from being improved in quality and produced in a steady manner.
In view of the above problem, the sheet hold-down means is introduced in the punch unit of the present invention for holding down the soft metal sheet when the sheet is punched, so that the soft metal sheet thus punched is prevented from being lifted by the punch after completion of the punching operation thereof, whereby the punch hole is improved in quality.
Preferably, in the punch unit of the present invention, the punch is provided with a cutting edge portion assuming a circularly-curved shape in its font end portion for improving the quality of the punch hole.
Heretofore, as shown in an upper portion of FIG. 7A, a conventional punch has been provided with a wedge-shaped cutting edge portion 2axe2x80x2 in its front end portion in order to reduce a punching load imposed on the punch in a punching operation of a workpiece. However, when the workpiece is punched by using such a conventional punch to form a punch hole, a pair of notches xe2x80x9cgxe2x80x9d are often formed in a peripheral portion of the punch hole to impair the quality of the punch hole, as shown in a lower portion of FIG. 7A. The cause found by the inventor of the present invention for such formation of the notches xe2x80x9cgxe2x80x9d in the peripheral portion of the punch hole is the presence of a wedge-shaped concave bottom portion xe2x80x9cexe2x80x9d in a front end portion of the conventional punch, which concave bottom portion xe2x80x9cexe2x80x9d of the conventional punch locally hits the workpiece at the end of the punching operation of the workpiece to produce these defective notches xe2x80x9cgxe2x80x9d. In view of the above, in the punch unit of the present invention, as shown in an upper portion of FIG. 7B, a circularly-curved cutting edge portion 2a (a radius of which is denoted by the reference letter xe2x80x9cRxe2x80x9d) is provided in a front end portion of its punch. Since the punch of the punch unit of the present invention has such circularly-curved cutting edge portion 2a in its front end, it is possible for the punch unit of the present invention to prevent the punch from locally hitting the workpiece (i.e., the soft metal sheet). This prevents the notches xe2x80x9cgxe2x80x9d from being formed in the peripheral portion of the punch hole formed in the soft metal sheet by using the punch unit of the present invention. Consequently, it is possible for the punch unit of the present invention to solve the above problem inherent in the conventional punch.
The punch unit of the present invention is most effectively used when the workpiece or soft metal sheet to be punched is made of aluminum or aluminum alloys.
As described above, the punch unit of the present invention is capable of punching a high-quality punch hole in the soft metal sheet, and also capable of remarkably increasing the service life of each of its punch and its die. Consequently, it is possible for the punch unit of the present invention to remarkably improve a punching operation of an aluminum sheet and the like in accuracy and productivity, which punching operation is required to produce a printing plate.